Generally, microfilter designs used in lab-on-a-chip (LOC) or micro total analysis system (μTAS) platforms can be categorized into four groups, namely 1) Weir, 2) pillar, 3) crossflow, and 4) membrane filters. A comparison of these different filter types is provided in Ji et al., “Silicon-based microfilters for whole blood cell separation,” Biomed Microdevices, 10(2):251-7 (April 2008) (hereinafter “Ji”). As indicated in Table 2 of Ji, a crossflow arrangement provides the highest efficiency in terms of its ability to pass red blood cells and trap white blood cells along with the greatest capacity to pass large volumes (see Table 1 of Ji), while the Weir and membrane filters are much more prone to clogging.
Gradient pillar array interfaces, an extension of the pillar-type filters, have also been found to be an effective means of pre-stretching DNA molecules. See, for example, U.S. Pat. No. 7,217,562 issued to Cao et al., entitled “Gradient Structures Interfacing Microfluidics and Nanofluidics, methods for Fabrication and Uses Thereof.” Gradient pillar array interfaces have found enhanced utility in staged filtering where particles are screened incrementally by size from largest to smallest to improve filter lifetime. See, for example, Wunderlich et al., “Microfluidic mixer designed for performing single-molecule kinetics with confocal detection on timescales from milliseconds to minutes,” Nature Protocols, vol. 8, no. 8, pgs. 1459-1474 (July 2013) (FIG. 1e shows a filter post array with rows of decreasing post separation). The primary problems with a pillar filter arrangement are area requirements, typically requiring wide reservoirs, and the fact that these filters form an abrupt entropic barrier that can lead to rapid fouling over the filter interface since pile up in one location rapidly leads to wide-spread clogging.
A key problem with the majority of these filter designs is that they utilize arrays of micro- or nanochannels of uniform width over some distance to filter particles which creates a large entropic barrier, leading even to filtering of particles that should be permitted to pass. This also makes the filters less efficient and more prone to clogging. This channel design is even utilized for many of the filters that are classified as pillar filters as well, where the pillars have a square geometry with a uniform gap between them. Other filters of the pillar-type refer to cylindrical pillars in a gradient arrangement. In this configuration, particle pill-up or fouling occurs at the interfaces between pillar arrays with dissimilar gap sizes and from the onset of a single interfacial clog a build-up along the interface rapidly propagates until useful fluidic flow ceases.
Therefore, improved micro- and nano-filter designs would be desirable.